1. Field of the Invention
The present invention relates to a passive matrix type or an active matrix type display device such as a liquid crystal display device, in particular, a fashionable display device having a large occupying area of a display portion on a substrate which is obtained by effectively forming a semiconductor integrated circuit for driving.
2. Description of the Related Art
Structures of a passive matrix type and an active matrix type have been known as a matrix type display device.
In the passive matrix type, a large number of strip type electrical wirings (row wirings) made of a transparent conductive film or the like are formed on a first substrate along a desired direction. Strip type electrical wirings (column wirings) are formed on a second substrate in a direction substantially perpendicular to the desired direction of the electrical wirings formed on the first substrate. These substrates are arranged so that the electrical wirings formed on both the substrates are opposite to one another.
An electro-optical material such as a liquid crystal material that has a transparent (transmittance) degree and a photoreflective-scattering degree which are changed by a voltage is formed between the substrates. The transparent degree, the photoreflective-scattering degree and the like in its intersection portion can be selected by applying (supplying) a voltage, a current or the like between a desired row wiring on the first substrate and a desired column wiring on the second substrate. Thus, a matrix display can be performed.
In the active matrix type, row wirings and column wirings are formed on the first substrate using a multilayer wiring (interconnection) technique. Pixel electrodes are formed in those portions where the row wirings and column wiring intersect. An active element such a thin film transistor (TFT) is formed at each pixel electrode, so that a structure which controls a voltage (potential) or a current with respect to the pixel electrodes is obtained. Also, a transparent conductive film is formed on the second substrate. The first and second substrates are arranged so that the pixel electrodes on the first substrate are opposite to the transparent conductive film on the second substrate.
A substrate is selected in accordance with a production process which will be used. In the passive matrix type, since a complex process is not performed except that a transparent conductive film is formed and then etched to form row and column wiring patterns, a glass substrate and a plastic substrate can be used. On the other hand, in the active matrix type, since a film formation process with relatively high temperature is performed and the prevention of an active ion such as sodium is required, it is necessary to use a glass substrate having an extremely low alkali concentration.
In a conventional matrix type display device, except for a special device, it is necessary to provide the display device with a semiconductor integrated circuit (a peripheral driver circuit or a bar circuit) for driving a matrix circuit. Such a driver circuit is mounted by using tape automated bonding (TAB) or chip on glass (COG). However, since the display device has a large scale matrix with, for example, about several hundred lines, the number of terminals in an integrated circuit is extremely large. Since a driver circuit is constructed by a rectangular-shaped integrated circuit (IC) package and a semiconductor chip, it is necessary to lead wirings in order to connect these terminals to electrical wirings on a substrate. Therefore, an area of peripheral portion cannot be neglected in comparison with a display screen. That is, this peripheral area is relatively large.
To solve the problem, a driver circuit has been formed on a long and narrow substrate (also called a stick or stick crystal) having substantially the same length as a side of a matrix circuit. The driver circuit is then connected to terminals of the matrix circuit. This technique is disclosed in Japanese Patent Application Open No. 7-14880. Since a width of about 2 mm is sufficient for the driver circuit, such arrangement is possible. Thus, an area on the substrate can be used as a display screen.
In this state, when a matrix circuit has a large a circuit cannot be formed on a silicon wafer, it is necessary to form the circuit on a glass substrate or the like. Thus, an active element disposed in the pixel electrode on a semiconductor circuit formed on a glass substrate or the like is a TFT using a crystalline semiconductor or an amorphous semiconductor.
With respect to the stick crystal, a thickness of a substrate for a driver circuit may suppress miniaturization of the whole display device. It is possible that a thickness of a substrate is 0.3 mm in order to obtain a thinner display device by optimizing the kind of substrate for the production process. Due to the strength required in a producing process, it is difficult to set a thickness of the stick crystal to 0.5 mm or less.
When the material of the stick crystal is different from that of the substrate of the display device, a defect may occur in a circuit by a difference of thermal expansion or the like. In particular, when a plastic substrate is used in the display device, such defects occur often. It is substantially impossible from a heat resistance view to use plastic as a substrate for the stick crystal. Since the formed semiconductor integrated circuit are thin, wirings to be connected to the semiconductor integrated circuit may be disconnected (broken) at a large step portion of end portions of the semiconductor integrated circuit. Also wiring resistance becomes high, so that a product yield of the whole device and reliability are reduced.
In the passive matrix type liquid crystal display device, a first plurality of strip type electrode wirings made of a transparent conductive film are provided on a first substrate and extended in a first direction. A second plurality of electrode wirings made of a transparent conductive film are provided on a second substrate and extended to a direction substantially perpendicular to the first direction. The first electrode wirings are provided to be opposite to the second electrode wiring through spacers scattered between the first and second substrates. A liquid crystal material is filled between the first and second electrode wirings and sealed by mainly a seal material (member) which is provided in around a region in which the first substrate is opposite to the second substrate. A peripheral driver circuit, which is connected to the first and second electrode wirings and controls pixels formed by these electrode wirings and the liquid crystal material, is provided outside the region that the first substrate is opposite to the second substrate.
In the passive matrix type liquid crystal display device, a complex process is not performed except that a transparent conductive film is formed on a substrate and then etched to form strip type electrical wirings. In addition, a temperature at which the substrate is to be processed is low. Thus, a glass substrate and a plastic substrate can be used as the first and second substrates.
In an active matrix driver type liquid crystal display device, a first substrate in which an active matrix circuit is provided is disposed to be opposite to a second substrate (an opposite substrate) that has an opposite transparent electrode provided on the whole surface, through spacers scattered on the first substrate. A liquid crystal material is sealed by mainly a seal material which is provided in periphery of a region that the first substrate is opposite to the second substrate. In the active matrix circuit, pixel electrodes connected to TFTs are disposed in a plurality of matrix forms. Outside the region that the first substrate is opposite to the second substrate, a source driver circuit and a gate driver circuit are provided as a peripheral driver circuit for driving the active matrix circuit.
In a conventional matrix type liquid display device, the peripheral driver circuit is formed by using a semiconductor integrated circuit and mounted by using TAB or COG. However, the number of electrode wirings for constructing a display screen is several hundreds or more. Since a driver circuit is an IC package and a semiconductor chip, it is necessary to lead wirings in order to connect these terminals to electrical wirings on a substrate. Therefore, an area of peripheral portion cannot be neglected in comparison with a display screen.
To solve the above problem, there is a method of forming a semiconductor integrated circuit using TFTs directly on a substrate except in a region where the first substrate is opposite to the second substrate and pixels are formed. Also, there is a method for obtaining the semiconductor integrated circuit by forming directly a driver circuit on a substrate on which a silicon thin film is deposited using an integrated circuit producing technique. In another method, a semiconductor integrated circuit using TFTs is formed on other supporting substrate by using the same technique, and then peeled to adhere it on the first and second substrates, or adhered to the substrate before removing an original supporting substrate.
In a liquid crystal display device having such a structure, it is necessary to provide a protective film made of an organic resin and a silicon nitride system substance in order to prevent the semiconductor integrated circuit from contaminating due to an impurity such as moisture, dust, sodium. However, when such a structure is used, stress due to the protective film acts to the TFTs constructing the semiconductor integrated circuit. Thus, a density of a recombination center of silicon in the TFT is increased and various characteristics such as threshold voltage of the TFT are changed. Also, a characteristic of the TFT constructing the semiconductor integrated circuit is changed by influence due to external pressure applied after the liquid crystal display device is completed.
To solve the above problem, an example of a conventional active matrix type liquid crystal display device is shown in FIG. 12. In FIG. 12, an active matrix circuit 305 including pixel electrodes (not shown), a source driver circuit 303 and a gate driver circuit 304 are provided on a first substrate 301. An opposite (counter) electrode opposite to the pixel electrodes is provided on a whole surface of a second substrate (counter substrate) 302. Spacers (not shown) are scattered on the first substrate 301. Between both electrodes a liquid crystal material 306 is filled and sealed by a seal material 307.
In FIG. 12, not only the active matrix circuit 305 but also the source driver circuit 303 and the gate driver circuit 304 which are a peripheral driver circuit are opposite to the counter substrate to be in contact with the liquid crystal material 306. That is, by the liquid crystal material 306, TFTs constructing the peripheral driver circuit are protected. This structure is disclosed in Japanese Patent Application Open No. 5-66413, for example.
In the liquid crystal display device, spacers which have a spherical shape, a stick shape, an angular shape or the like between the substrates and are made of a hard material such as silica are scattered uniformly, to maintain an interval between two substrates. Each spacer has a diameter corresponding to the same length as an interval between the substrates. The diameter is about 3 μm to 8 μm in a display device using a nematic liquid crystal, and 1 μm to 4 μm in a display device using a smectic liquid crystal. The number of the spacers is about 50 to 1000 per one pixel in a case wherein a size of one pixel is several 10 μm square to several 100 μm square.
In the peripheral driver circuit, a large number of TFTs are provided extremely close to one another. Thus, in the liquid crystal display device of FIG. 12, since the peripheral driver circuit is provided within a liquid crystal region, if external stress is applied to the substrates, the peripheral driver circuit may be broken by the spacers provided between the substrates. Thus, the peripheral driver circuit do not operate regularly, a point defect and a line defect occur and further a display may be impossible, so that reliability and durability of the liquid crystal display device are reduced. Such a phenomenon occurs remarkably in the liquid crystal display device using a plastic substrate which is modifiable by external stress.